1. Cross-Reference
The present application is the United States counterpart of earlier-filed Japanese patent application no. 1-310593, filed Dec. 1, 1989, which application is incorporated herein by reference and to which application applicants claim priority under 35 USC .sctn.119.
2. Field of the Invention
The present invention relates generally to the field of anti-skid braking systems. More particularly, it relates to failure detection circuits for use in such systems.
3. Background Art
Various anti-skid control devices have been developed to prevent the wheels of a vehicle from locking or skidding during braking. In general terms, these devices function by controllably modulating the amount of braking action (via modulating brake fluid pressure) at each of the four wheels by using valves and electronic control means.
An anti-skid control device generally consists of wheel speed detecting means located at each of the wheels, a computational arithmetic means capable of calculating the slip ratio based on the wheel speed and acceleration, and a control means to control (modulate) the amount of brake fluid pressure applied to each of the wheel brakes based on the output signal from the computational arithmetic means. Such devices are capable of facilitating the deceleration of a vehicle while regulating the wheel deceleration speed within an appropriate slip angle ratio by controlling the braking pressure according to the slip conditions of the wheel. When functioning properly, the presence of an anti-skid braking system is generally indetectable to the driver. That is, the driver is unaware that the anti-skid braking system is present. Anti-skid braking systems have substantial advantages in preventing wheel skid and allowing the application of brakes during turning motions and on surfaces of variable coefficient of friction. It is very important that a driver be informed if there is an inadvertent failure in the anti-skid braking system. Although, typically, the failure of the anti-skid system will lead to normal (no anti-skid) braking action, the advantages and safety margin provided by the anti-skid system will be missing. Thus it is important that the driver be alerted to the failure of the anti-skid system. It is known in the art to provide anti-skid braking systems with a failure detection circuit to send a signal to the driver and, at the same time, cause the anti-skid braking system to revert to a normal braking system.
Typical failure detection circuits used in the past comprise a voltage monitoring circuit monitoring the integrity of the electronic functions in the anti-skid braking system, a physical failure detection circuit monitoring the physical integrity of the pressurized brake fluid control circuit, electromagnetic valves, etc., a main power source circuit breaker which shuts down the system in case of a failure, and a means for providing a warning if any of these systems detects a failure. Two power feed circuits are ideally required for the means for warning. This redundancy allows the means for warning to continue to send a warning signal even if the failure that the system has detected has occurred in the primary electrical power feed to the detection system.
Although these background systems are well understood, there are a number of disadvantages inherent in them. These inadequacies of prior systems include:
(1) When a failure occurs in the main power source circuit breaker or in the main power source circuit, current from the vehicle will be permitted to flow to the warning circuit through the remainder of the two above-mentioned power feed circuits. Accordingly, the failure in the main power source breaker or the main power source circuit would be difficult to diagnose and correct.
(2) When a failure occurs in the main power source circuit breaker or the main power source circuit itself, in conventional systems this will lead to an increase in consumption of electric current, but since the failure is in the main power source circuit breaker or in the main power source circuit itself, this increase in current may not be detected if the available current is not overtaken by demand and no voltage drop occurs.
(3) The above two problems can be solved by installing a separate dedicated failure-detecting device to detect specifically a failure in the main power source breaker and main power supply circuit. However, such a device requires intricate circuit structure and can lead to high manufacturing costs.